This invention relates to sealing wheels for webs of sheet material such as used in packaging machines to seal the margins of superimposed travelling webs which are to form packages. In the past, such sealing wheels have in some cases been mounted in opposed pairs above and below the sheet material to be sealed, and have been arranged to exert a suitable sealing pressure. In some cases, one or more of the wheels has been heated to provide a heat seal and, in other cases, pressure sensitive adhesive has been used. This required pressure but not heat. In both cases, difficulty has been encountered due to minor abnormalities in the position and/or dimensions of one or both sealing wheels. For example, the shaft supporting one of the sealing wheels might bow slightly to cause one wheel to be slightly farther away from the work than the others. This may occur when several axially spaced sealing wheels are supported on the same shaft. Also, the periphery of one or more of the sealing wheels might be sufficiently eccentric to cause intermittent interruption or minor reduction in the sealing pressure along the line of seal, thus affecting the critical pressure-temperature-time relationship. Since a dimensional or positional abnormality as small as 0.001 inches can cause a defective seal, this presents a serious problem in packages used for the health industry or for food or anhydrous materials, as openings which might admit contaminating foreign matter, air or moisture would seriously affect the sterility, condition or usability of the contents.